arXiv subjects

Chris H. Greene

Publications and source records attributed to Chris H. Greene.

At least 37 records · Page 2Linked to original sources

arXiv2023

Observation of coherent oscillations in association of dimers from a thermal gas of ultracold atoms

We report the observation of coherent oscillations in conversion efficiency of weakly-bound dimers formed from a thermal gas of ultracold atoms. Finite thermal energy of the gas causes loss of coherence when a broad continuum is resonantly coupled to a discrete bound state. Restoration of the coherence can be achieved through non-adiabatic transitions of the dressed molecular energy level that are induced by a strong modulation pulse with fast envelope dynamics. Conditions to observe coherent oscillations are verified, and control of their properties is demonstrated. The main experimental findings are supported by theoretical modeling and numerical calculations. The observed results may lead to a renewed interest in general studies of a discrete energy level coupled to a broadband continuum when the properties of both are fully controlled.

cond-mat.quant-gas↗

arXiv2023

Coherent control phase lag across doubly-excited atomic strontium resonances in an $ω-2ω$ interference scheme

Accurate calculations of phase lag associated with coherent control where an excited system decays into more than one product channel have recently been reported for atomic barium in PhysRevA.105.013113 (2022). The present study extends the calculations to make predictions of that observable for another alkaline earth atom, strontium, with a discussion of its spectrum and phase lag for energies between the $Sr^{+}$ $4d_{3/2}$ and $4d_{5/2}$ thresholds. We will explore the physics influenced by the electron correlations of strontium and the long-range Coulomb potential. The behavior of the phase lag cannot be simply addressed by a time delay analysis, although the latter is often used to address the prominent channel of resonance decay of doubly excited states.

physics.atom-ph↗

arXiv2023

Unified treatment of resonant and non-resonant mechanisms in dissociative recombination: benchmark study of CH$^+$

The theoretical approach developed here treats uniformly the direct and indirect mechanisms of dissociative recombination (DR) in a diatomic ion. The present theory is based on electron scattering calculations performed at several internuclear distances in the molecule. It is easy to implement becaus there is no need to separately evaluate couplings and the bound dissociative states of the neutral molecule. The theory can be applied to molecular ions with or without electronic resonances at low energies. The approach is applied to compute the DR cross section in electron-CH$^+$ collisions. The computed cross section agrees generally well with recent state-resolved data from a cryogenic storage-ring experiment, which validates the approach.

physics.atom-ph↗

arXiv2022

Three and four identical fermions near the unitary limit

This work analyzes the three and four equal-mass fermionic systems near and at the $s$- and $p$-wave unitary limits using hyperspherical methods. The unitary regime addressed here is where the two-body dimer energy is at zero energy. For fermionic systems near the $s$-wave unitary limit, the hyperradial potentials in the $N$-body continuum exhibit a universal long-range $R^{-3}$ behavior governed by the $s$-wave scattering length alone. The implications of this behavior on the low energy phase shift are discussed. At the $p$-wave unitary limit, the four-body system is studied through a qualitative look at the structure of the hyperradial potentials at unitarity for the $L^π=0^{+}$ symmetry. A quantitative analysis shows that there are tetramer states in the lowest hyperradial potentials for these systems. Correlations are made between these tetramers and the corresponding trimers in the two-body fragmentation channels. Universal properties related to the four-body recombination process $\mathrm{A+A+A+A}\leftrightarrow \mathrm{A_3+A}$ are discussed.

physics.atom-ph↗

arXiv2022

Triatomic Photoassociation in an Ultracold Atom-Molecule Collision

Ultracold collisions of neutral atoms and molecules have been of great interest since experimental advances enabled the cooling and trapping of such species. This study is a theoretical investigation of a low-energy collision between an alkali atom and a diatomic molecule, accompanied by absorption of a photon from an external electromagnetic field. The long-range interaction between the two species is treated, including the atomic spin-orbit interaction. The long-range potential energy curves for the triatomic complex are calculated in realistic detail, while the short-range behavior is mimicked by applying different boundary conditions at the van der Waals length. The photoassociation (PA) rate of an atom colliding with a dimer is calculated for different alkali atoms, namely Na and Cs. The model developed in this study is also tested against known results for the formation rate of the Cs$_3$ complex via PA, namely to compare with the work done by Rios et al., PRL 115, 073201 (2015), and the results are in generally good agreement.

physics.atom-ph↗

arXiv2022

Attosecond Entangled Photons from Two-Photon Decay of Metastable Atoms: A Source for Attosecond Experiments and Beyond

We propose the generation of attosecond entangled bi-photons in the extreme-ultraviolet regime by two-photon decay of a metastable atomic state as a source similar to spontaneous parametric down-conversion photons. The 1s2s $^1S_0$ metastable state in helium decays to the ground state by emission of two energy-time entangled photons with a photon bandwidth equal to the total energy spacing of 20.62 eV. This results in a pair correlation time in the attosecond regime making these entangled photons a highly suitable source for attosecond pump-probe experiments. The bi-photon generation rate from a direct four photon excitation of helium at 240 nm is calculated and used to assess some feasible schemes to generate these bi-photons. Possible applications of entangled bi-photons in attosecond time scale experiments, and a discussion of their potential to reach the zeptosecond regime are presented.

quant-ph↗

arXiv2022

Analyzing the Rydberg-based omg architecture for $^{171}$Yb nuclear spins

Neutral alkaline earth(-like) atoms have recently been employed in atomic arrays with individual readout, control, and high-fidelity Rydberg-mediated entanglement. This emerging platform offers a wide range of new quantum science applications that leverage the unique properties of such atoms: ultra-narrow optical "clock" transitions and isolated nuclear spins. Specifically, these properties offer an optical qubit ("o") as well as ground ("g") and metastable ("m") nuclear spin qubits, all within a single atom. We consider experimentally realistic control of this "omg" architecture and its coupling to Rydberg states for entanglement generation, focusing specifically on ytterbium-171 ($^{171}\text{Yb}$) with nuclear spin $I = 1/2$. We analyze the $S$-series Rydberg states of $^{171}\text{Yb}$, described by the three spin-$1/2$ constituents (two electrons and the nucleus). We confirm that the $F = 3/2$ manifold -- a unique spin configuration -- is well suited for entangling nuclear spin qubits. Further, we analyze the $F = 1/2$ series -- described by two overlapping spin configurations -- using a multichannel quantum defect theory. We study the multilevel dynamics of the nuclear spin states when driving the clock or Rydberg transition with Rabi frequency $Ω_\text{c} = 2 π\times 200$ kHz or $Ω_\text{R} = 2 π\times 6$ MHz, respectively, finding that a modest magnetic field ($\approx200\,\text{G}$) and feasible laser polarization intensity purity ($\lesssim0.99$) are sufficient for gate fidelities exceeding 0.99.

physics.atom-ph↗

arXiv2022

Multichannel photoelectron phase lag across atomic barium autoionizing resonances

Phase lag associated with coherent control where an excited system decays into more than one product channel has been subjected to numerous investigations. Although previous theoretical studies have treated the phase lag across resonances in model calculations, quantitative agreement has never been achieved between the theoretical model and experimental measurements of phase lag from the $ω-2ω$ ionization of atomic barium \cite{PhysRevLett.98.053001,PhysRevA.76.053401}, suggesting that a toy model with phenomenological parameters is inadequate to describe the observed phase lag behavior. Here the phase lag is treated quantitatively from a multichannel coupling formulation, and our calculations based on multichannel quantum defect and $R$-matrix treatment achieves good agreement with the experimental observations. Our treatment also develops formulas to describe the effects of hyperfine depolarization on multiphoton ionization processes, and further, identifies resonances between $Ba^{+}$ $5d_{3/2}$ and $5d_{5/2}$ thresholds that have apparently never been experimentally observed and classified.

physics.atom-ph↗

arXiv2021

Competing Ionization and Dissociation in the H$_2$ Gerade System

A numerically solvable two-dimensional (2D) model, employed by the authors to study the dissociative recombination of H$_2^+$ in the ungerade symmetry [Phys. Rev. A $\mathbf{98}$, 062706 (2018)], is extended to describe the collision process in the gerade symmetry of H$_2$. In this symmetry the ionization and dissociation processes are driven primarily by the direct, curve-crossing mechanism. The model is represented by a set of three coupled electronic channels in 2D, in the space of $s,p,d$ partial waves of the colliding electron. We demonstrate that the Born-Oppenheimer properties of the H$_2$ molecule in the relevant range of internuclear distances can be described by such a model. The molecular rotational degrees of freedom are accounted for by the rotational frame transformation. The numerical solution of the model is discussed and the resulting rovibrationally inelastic and dissociative recombination cross sections are compared with the available data.

physics.chem-ph↗

arXiv2021

Asymmetric lineshapes of Efimov resonances in mass-imbalanced ultracold gases

The resonant profile of the rate coefficient for three-body recombination into a shallow dimer is investigated for mass-imbalanced systems. In the low-energy limit, three atoms collide with zero-range interactions, in a regime where the scattering lengths of the heavy-heavy and the heavy-light subsystems are positive and negative, respectively. For this physical system, the adiabatic hyperspherical representation is combined with a fully semi-classical method and we show that the shallow dimer recombination spectra display an asymmetric lineshape that originates from the coexistence of Efimov resonances with Stückelberg interference minima. These asymmetric lineshapes are quantified utilizing the Fano profile formula. In particular, a closed form expression is derived that describes the width of the corresponding Efimov resonances and the Fano lineshape asymmetry parameter $q$. The profile of Efimov resonances exhibits a $q-$reversal effect as the inter- and intra-species scattering lengths vary. In the case of a diverging asymmetry parameter, i.e. $|q|\to \infty$, we show that the Efimov resonances possess zero width and are fully decoupled from the three-body and atom-dimer continua, and the corresponding Efimov metastable states behave as bound levels.

cond-mat.quant-gas↗

arXiv2021

Efimov physics implications at $p$-wave fermionic unitarity

Efimov physics at $p$-wave unitarity for three equal mass fermions in multiple symmetries interacting via Lennard-Jones potentials is predicted to modify the long range interaction potential energy, but without producing a true Efimov effect. This analysis treats the following total orbital angular momenta and parities, $J^Π=0^{+}, 1^{+}, 1^{-}$ and $2^{-}$, for either three spin-polarized fermions ($\uparrow \uparrow \uparrow $), or two spin-up and one spin-down fermion ($\downarrow \uparrow \uparrow $). Our results for the long range interaction in some of those cases agree with previous work by Werner and Castin and by Blume {\it et al.}, namely in cases where the $s$-wave scattering length goes to infinity. The present results extend those calculated interaction energies to small and intermediate hyperradii comparable to the van der Waals length, and we consider additional unitarity scenarios where the $p$-wave scattering volume approaches infinity. The crucial role of the diagonal hyperradial adiabatic correction term is identified and characterized.

physics.atom-ph↗

arXiv2021

Resonant control of photoelectron directionality by interfering one- and two-photon pathways

Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground state helium atom above-threshold photoionization for energies up to the $N=2$ threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $ΔΦ$, the reduced electric field strength $χ=\mathcal{E}_ω^2/{\mathcal{E}_{2ω}}$, and the final state energy $ε$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where $90\%$ of photoelectrons whose energy is near the $2p^2\ ^1S^e$ resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.

physics.atom-ph↗

arXiv2021

Two-photon Above Threshold Ionization of Helium

Multiphoton ionization provides a clear window into the nature of electron correlations in the helium atom. In the present study, the final state energy range extends up to the region near the $N=2$ and $N=3$ ionization thresholds, where two-photon ionization proceeds via continuum intermediate states above the lowest threshold. Our calculations are performed using multichannel quantum defect theory (MQDT) and the streamlined R-matrix method. The sum and integration over all intermediate states in the two-photon ionization amplitude is evaluated using the inhomogeneous R-matrix method developed by Robicheaux and Gao. The seamless connection of that method with MQDT allows us to present high resolution spectra of the final state Rydberg resonances. Our analysis classifies the resonances above the $N=2$ threshold in terms of their group theory quantum numbers. Their dominant decay channels are found to obey the previously conjectured propensity rule far more weakly for these even parity states than was observed for the odd-parity states relevant to single photon ionization.

physics.atom-ph↗

arXiv2020

Orbital Variational Adiabatic Hyperspherical Method Applied to Bose-Einstein Condensates

A variational basis set motivated by mean-field theory is utilized to describe the Bose-Einstein condensate within the adiabatic hyperspherical coordinate framework. The simplest single-orbital variant of this treatment reproduces many of the ground state properties predicted by the Gross-Pitaevskii equation. But a multi-orbital improvement to the basis set yields a better representation of particle correlations and of the critical number where the condensate collapses for a negative two-body scattering length. The method also produces systematic deviations from Bogoliubov theory for the fundamental monopole excitation frequency.

physics.atom-ph↗

arXiv2020

A Comprehensive Study of the Three- and Four-Neutron Systems at Low Energies

This work presents further analysis of the three- and four-neutron systems in the low energy regime using adiabatic hyperspherical methods. In our previous Phys. Rev. Lett. article (Phys. Rev. Lett. 125, 052501 (2020)), the low-energy behavior of these neutron systems was treated in the adiabatic approximation, neglecting the off-diagonal non-adiabatic couplings. A thorough analysis of the density of states through a multi-channel treatment of the three-and four-neutron scattering near the scattering continuum threshold is performed, showing no evidence of a 4n resonance at low energy. A detailed analysis of the long-range behavior of the lowest few adiabatic hyperspherical potentials shows there is an attractive $ρ^{-3}$ universal behavior which dominates in the low-energy regime of the multi-channel scattering. This long-range behavior leads to a divergent behavior of the density of state for $E\rightarrow0$ that could account for the low-energy signal observed in the 2016 experiment by Kisamori et al. (Phys. Rev. Lett. 116, 052501 (2016)).

nucl-th↗

arXiv2020

Counterexample to the Bohigas Conjecture for Transmission Through aOne-Dimensional Lattice

Resonances in particle transmission through a 1D finite lattice are studied in the presence of a finite number of impurities. Although this is a one-dimensional system that is classically integrable and has no chaos, studying the statistical properties of the spectrum such as the level spacing distribution and the spectral rigidity shows quantum chaos signatures. Using a dimensionless parameter that reflects the degree of state localization, we demonstrate how the transition from regularity to chaos is affected by state localization. The resonance positions are calculated using both the Wigner-Smithtime-delay and a Siegert state method, which are in good agreement. Our results give evidence for the existence of quantum chaos in one dimension which is a counter-example to the Bohigas-Giannoni-Schmit conjecture.

nlin.CD↗

arXiv2020

Observability of a Sharp Majorana Transition in a Few-Body Model

We propose experimentally observable signatures of of topological Majorana quasiparticles in the few-body limit of the interacting cold-atom model of [Iemini et al. Phys. Rev. Lett. 118 200404 (2017)]. In this limit, the total on-site density and single-body correlations change smoothly with the model parameters, while the calculated mutual information of opposite ends of the lattice indicates a sharp transition of the system to a topological ground state. Furthermore, local density and parity measurements provide an experimentally viable path for observing the ground state Majorana quasiparticles in ultracold atoms. Our results lay out a promising future for utilizing few-body systems as a testing ground for Majorana physics.

quant-ph↗

arXiv2020

Collisional spin transfer in an atomic heteronuclear spinor Bose gas

We observe spin transfer within a non-degenerate heteronuclear spinor atomic gas comprised of a small $^7$Li population admixed with a $^{87}$Rb bath, with both elements in their $F=1$ hyperfine spin manifolds and at temperatures of 10's of $μ$K. Prepared in a non-equilibrium initial state, the $^7$Li spin distribution evolves through incoherent spin-changing collisions toward a steady-state distribution. We identify and measure the cross-sections of all three types of spin-dependent heteronuclear collisions, namely the spin-exchange, spin-mixing, and quadrupole-exchange interactions, and find agreement with predictions of heteronuclear $^7$Li-$^{87}$Rb interactions at low energy. Moreover, we observe that the steady state of the $^7$Li spinor gas can be controlled by varying the composition of the $^{87}$Rb spin bath with which it interacts.

physics.atom-ph↗